Across Belgium, Finland, France, Germany, Greece, Italy, Spain, Sweden and the United Kingdom, right-wing terrorists were responsible for 5 failed, foiled and completed attacks in 2017 - out of a total of 205. That's 2.4% of the total.
Separatist, jihadist and left-wing terrorism were responsible for 137 (67%), 33 (16%) and 24 (12%) failed, foiled and completed attacks.
Clearly, for all the moral panic about the "threat" of the "far right", it is actually the least significant source of terrorism.
Notably, left-wing terrorists were responsible for 5 times as many attacks. Which might explain why the left obsesses so much about the "far right" "threat".
There're also tables for arrests and verdicts by affiliation, but unfortunately, the UK does not report affiliation. Given that it represents a significant proportion of the arrests and verdicts, any conclusions would be caveated.
Source: EUROPEAN UNION TERRORISM SITUATION AND TREND REPORT 2018 (TESAT 2018) | Europol